Affinity chromatography is a common method for concentrating and separating target molecules from a sample, and is based on specific, three-dimensional interactions between target molecules and entities to which they bind (i.e., ligands). Ligands can be isolated or generated for binding to practically any target molecule. Potential ligands include biological molecules such as proteins, antibodies, peptides, and the like. Libraries of millions of potential ligands are generated using combinatorial synthesis techniques, many of which are well known in the art (see, for example, Lam et al., Nature, 354, 82–84 (1991) and International Patent Application WO 92/00091). To aid in separation of target molecules from a sample, ligands can be affixed to a solid support matrix, such as individual particles (e.g., chromatography resin beads) or contiguous supports (e.g., arrays). Ligands immobilized on a solid support matrix can then be employed to purify targets from complex solutions (Baumbach and Hammond, BioPharm May, 24–31 (1992)).
Aside from mere separation of target molecules from a sample, affinity chromatography provides a means of examining binding interactions between potential ligands and target molecules. Detecting target-ligand binding can be challenging. Techniques for detecting ligand-target complexes have been developed, e.g. radiolabelling and immunological methods (see, for example, U.S. Pat. No. 5,834,318, International Patent Application WO 01/40265, and U.S. patent application Ser. No. 09/453,115). However, currently available detection techniques have potential drawbacks, namely modification of the target through radiolabel attachment, interference from ligand-target-detection system interactions, and the ability to detect only a limited number of targets for which detection systems already exist.
In view of the above, there remains a need in the art for a method of detecting target-ligand binding that avoids modification of the target for detection, enables detection of a target separated from a ligand and identification of the ligand, and allows detection of the target via its biological, biochemical or chemical activity. The invention provides such a method. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.